Part fixturing is a critical and expensive process in the machining of parts. In all high volume machining systems, dedicated fixtures are used to hold parts. However, the fixture also serves to position the workpiece in a known location so that machined features on the workpiece can be positioned with respect to a known datum. In past practices, this is accomplished by putting the part against fixture locators. The passive elements of the fixture are prescribed for a given workpiece and must be mechanically altered if the workpiece changes. The fixture typically clamps and locates the workpiece with respect to the machine tools that will conduct the metal removal process. The fixture acts as a dedicated part location system, prescribing the relationship between the workpiece datum and the machine tool coordinate system. Automotive experts consider the dedicated nature of fixturing the main barrier to the implementation of truly flexible machining systems for automotive parts, especially parts which are machined in high volumes.
Previous efforts in designing and developing flexible fixturing for either small batch manufacture or mass production scenarios can be generally divided into two groups: modular fixtures and conformable fixtures. Modular fixturing originated in the post-war era and consists of fixtures assembled from a standard library of elements such as V-blocks, toggle clamps, locating blocks, etc. Their flexibility lies in the ability to be reconfigured either manually or by a robotic device.
However, the modular fixtures have no intrinsic ability to adapt to different sizes and shapes of parts within a part family. In addition, the time necessary for reconfiguration is long, and modular fixtures generally lack stiffness. Consequently, modular fixtures appear to be more suited to a job shop environment than mass production.
The advent of Flexible Manufacturing Systems (FMS) in the early 1980's provided the impetus for work on conformable fixturing. A conformable fixture is defined as one that can be configured to accept parts of varying shape and size. Conformable fixture technology can be classified as encapsulant or mechanistic. Examples of encapsulant fixtures are found in the aerospace industry, where low melting-point metal is used to enclose turbine blades, and produce well-defined surfaces for part location and clamping for grinding operations. While an excellent means of facilitating the holding of complex parts, encapsulation is a costly and time consuming process, and is unsuitable for fixturing parts.
Mechanistic fixtures reported in the literature include the use of petal collets, programmable conformable clamps, a programmable/multi-leaf vise, and an adjustable integral fixture pallet. Of the four, the adjustable integral fixture pallet concept appears to be the most capable of accommodating a part family of castings. To date, however no feasibility studies have been conducted regarding the applicability of any of these techniques to production machining operations.
The need to reduce cycle times, partially through more aggressive cutting, has increased the forces that act on the part during machining. This drive for productivity has come at the same time that part geometry has often become more complex, with highly compliant part features and tighter tolerances. This has led to increased problems with fixture-part deformation and vibration.
A number of technologies currently exist that have the potential to be developed into a part location system. These are classified as either contact or noncontact technologies. Contact technologies include touch trigger probes and analog probes. Touch trigger probes are widely used with coordinate measurement machines for performing part metrology. There are also reported industrial applications in which a touch trigger probe is used to define datums on a workpiece for the purpose of alignment identification and tool path compensation. Unfortunately a touch trigger probe can only acquire data at a very low rate. Analog touch probes are capable of greater data acquisition rates, but to date, there have been no reported uses of analog probes for the purpose of datum establishment in machining applications.
Non-contact technologies include computer vision, laser probes, and laser scanning. Computer vision has been widely used in the electronics industry as a fast and accurate means to locate printed circuit boards for the purpose of board assembly. In these applications, the board location problem is strictly two dimensional. If multiple cameras are used, stereographic images of a part surface can be obtained. From such an image, it could be possible to establish a datum.
Laser probes are spindle-held units that operate on the principle of triangularization. Under carefully controlled environmental conditions, laser probes are capable of submicron accuracy. However, it is also known that the accuracy of these systems can degrade if the texture of the surface is poor or if ambient light conditions are poor. Laser scanners are similar to laser probes with the exception that they consist of either an array of laser probes or laser light emitters and receptors in the shape of lines or grid patterns. By using an array, the data acquisition time can be reduced and/or the data density can be increased.